Support 2019.2 deprecation replacement UI theme keys (#87)

The latest JetBrains product versions 2019.2 deprecated some UI them
keys in favour of new keys. This broke the styles when using the Nord
plugin with the latest product versions due to the missing new keys.
In order to support both versions `<2019.2` as well as `>=2019.2` the
new keys have been added while also keeping the deprecated ones until
they'll be removed from the UI theme API.

1. Editor tabs using new keys for the active background
   (`underlinedTabBackground`) and when hovered (`hoverMaskColor`) and
   also now support customization of the bottom stripe when not focused
   (`inactiveUnderlineColor`).
2. Debugger tabs are now using a new key for the background color
   (`underlinedTabBackground`) when active.
3. The auto-completion popup now uses the same color like the
   `DOCUMENTATION_COLOR` attribute of the editor color scheme as well as
   a new key for the currently active selection (`selectionBackground`).
4. The components of the status bar now using a new key to for the
   background color when hovered (`hoverBackground`).

Resolves GH-86

resources/themes/nord.xml

@@ -22,7 +22,7 @@
     <option name="DIAGRAM_NOTE_BACKGROUND" value="434c5e" />
     <option name="DIAGRAM_NOTE_BORDER" value="434c5e" />
     <option name="DIAGRAM_REALIZATION_EDGE" value="a3be8c" />
-    <option name="DOCUMENTATION_COLOR" value="434c5e" />
+    <option name="DOCUMENTATION_COLOR" value="3b4252" />
     <option name="ERROR_HINT" value="434c5e" />
     <option name="FILESTATUS_ADDED" value="a3be8c" />
     <option name="FILESTATUS_DELETED" value="bf616a" />

src/nord.theme.json

@@ -82,8 +82,10 @@
     "CompletionPopup": {
       "background": "#323846",
       "matchForeground": "#88c0d0",
-      "nonFocusedState": "#2e3440",
       "matchSelectionForeground": "#88c0d0",
+      "nonFocusedMask": "#2e3440",
+      "nonFocusedState": "#2e3440",
+      "selectionBackground": "#4c566a",
       "selectionInactiveBackground": "#3b4252",
       "selectionInactiveInfoForeground": "#4c566a",
       "selectionInfoForeground": "#eceff4"
@@ -103,7 +105,8 @@
       "foreground": "#2e3440"
     },
     "DebuggerTabs": {
-      "selectedBackground": "#3b4252"
+      "selectedBackground": "#3b4252",
+      "underlinedTabBackground": "#3b4252"
     },
     "DragAndDrop": {
       "areaBackground": "#2e3440",
@@ -118,10 +121,14 @@
     },
     "EditorTabs": {
       "borderColor": "#2e3440",
+      "hoverMaskColor": "#4c566a",
+      "inactiveColoredFileBackground": "#2e3440",
       "inactiveMaskColor": "#3b4252",
+      "inactiveUnderlineColor": "#616e88",
       "selectedBackground": "#4c566a",
       "selectedForeground": "#d8dee9",
-      "underlineColor": "#88c0d0"
+      "underlineColor": "#88c0d0",
+      "underlinedTabBackground": "#4c566a"
     },
     "FileColor": {
       "Blue": "#88c0d01a",
@@ -303,6 +310,7 @@
       "foreground": "#88c0d0"
     },
     "StatusBar": {
+      "hoverBackground": "#4c566a",
       "borderColor": "#3b4252"
     },
     "TabbedPane": {